"cron-like expression" in backup task

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1

I’m trying to configure “0 9,21 * * *” as the time in a daily backup task, but it is not accepted. Even “0 21 * * *”, “0 0 21 * * *” or “0 0 21 * * * *” are not accepted. Everytime I get an error “Cron expression is invalid”. What would be a valid expression here?

2

The quartz scheduler uses its own cron format - Cron Trigger Tutorial